The present invention relates to imaging systems. The present invention particularly relates to aerial reconnaissance systems and methods.
Various known teachings which are believed to be related to various ones of the innovations disclosed herein will now be discussed. However, it should be noted that not every idea discussed in this section is necessarily prior art. For example, characterizations of particular patents or publications may relate them to inventive concepts in a way which is itself based on knowledge of some of the inventive concepts. Moreover, the following discussion attempts to fairly present various suggested technical alternatives (to the best of the inventor's knowledge), even though the teachings of some of those technical alternatives may not be "prior art" under the patent laws of the United States or of other countries.
In aerial reconnaissance, the cost of each reconnaissance run is relatively large. It is therefore desirable to obtain a large amount of information from each run.
In one class of aerial systems, the motion of the air vehicle provides a scanning motion in one direction. Thus, it has long been recognized that a continuous sequence of linear images of the ground provides a convenient way to assemble a large-area image. In such a system, one of the determinants of image resolution is the lateral resolution within the image track.
Effects of Platform Motion
In a fixedly mounted reconnaissance system, any change in the vehicle's attitude will cause a corresponding movement of the image on the focal plane. This movement may be very large. Thus, reconnaissance under turbulent air conditions presents some inherent difficulty in attaining high resolution output, since the aerodynamically-caused movement of the vehicle may degrade resolution of the image. For example, in a system which is operating at 5000 feet, and which has a lateral resolution of 12,000 pixels across a 45 degree field of view, a roll axis attitude change of only one degree would cause a lateral image shift across several hundred pixels. Thus very small attitude changes have the potential to cause drastic shifts in the image.
It has therefore been recognized in the art that compensation for aircraft attitude changes is highly desirable. One previously suggested way to accomplish this is to use a camera (or electro-optic imager) assembly which is mounted on gimbals, and mechanically driven for stabilization. Another approach, used with film-based photographic systems, has been to actively move the film magazine. See, for example, U.S. Pat. Nos. 3,092,687, 3,503,663, 3,638,502, and 3,982,255, which are hereby incorporated by reference. However, many such approaches have the disadvantage that the moving mass is relatively large. This means that high frequency components of instability cannot be optimally compensated. Moreover, a mechanism which is able to move the whole assembly is likely to be relatively bulky, heavy, and expensive, particularly in view of the environmental constraints on such an assembly.
Turbulence is particularly likely to cause transient instabilities along the roll and pitch axes of an aircraft. Thus, one problem with prior methods is that the available quality of aerial reconnaissance has been dependent on air turbulence conditions. This is particularly a problem in military applications, where information may be needed urgently.
In reconnaissance systems for military applications, it is highly desirable for the air vehicle in which the reconnaissance equipment is mounted to be able to take evasive maneuvers. In many previous aerial reconnaissance systems, an aircraft running a reconnaissance mission must be very restrained in maneuvering. This makes it a better target. If the changes in the air vehicle's attitude cannot be performed without interrupting the reconnaissance run, then the air vehicle's survival and its mission are inherently in conflict, which is not an ideal situation. Thus, it would be highly desirable if aircraft (or other air vehicles) could maneuver with more freedom during reconnaissance runs. This would also be advantageous for non-military aerial surveying applications, since there would be less need for precise control of the aircraft course track during the surveying run.
One conventional type of reconnaissance system forms a line image onto a moving roll of film. Such systems inherently provide a slight degree of resistance to roll instabilities, since to some degree the effect of roll instabilities would show up merely as a waviness of the lines along the track of the airplane. However, such systems do not fully compensate even for roll instabilities, since, depending on the exposure time of the film (determined by the width of the slit and the speed of film transport), there still may be some blurring of detail. Moreover, such systems are vulnerable to pitch axis instabilities. Moreover, even if all of the information is present on the film, image interpretation may still be difficult if the image is distorted.
In addition to roll instabilities, air vehicles will also commonly have pitch instabilities of large magnitude. Pitch instabilities due to turbulence, or changes in pitch attitude due to maneuvering, are particularly likely to occur at a relatively high angular change rate, and therefore have a large potential to degrade imaging performance.
Yaw variations (i.e. rotations of the air vehicle around the geometric axis which would be vertical during normal level flight) can also be caused by turbulence, although the magnitude of these instabilities will typically be much smaller than those about the pitch and roll axes. However, yaw variations are an essential component of maneuvering an aircraft. Therefore, reconnaissance during maneuvering would be impractical without some way to compensate for yaw attitude changes.
An airplane may also have a "crab" component, where the track vector (in the ground frame of reference) is not perfectly aligned with the principal forward axis of the air vehicle. This will commonly occur where an airplane is flying in a cross-wind, and may also be caused by aerodynamic imbalance in a damaged aircraft. This component of motion will appear, at the focal plane, as a fixed offset or slowly varying component of yaw attitude. At moments when the cross-wind (at the plane's location) changes rapidly, there will in fact be a transient yaw component.
Electro-Optic Reconnaissance
In electro-optic systems, an optical train images ground features onto an imager, and the imager measures the image intensity at a number of locations. (Each such location is referred to as a picture element, or "pixel. ")
There are significant potential advantages to using electro-optic sensing methods in aerial reconnaissance applications. However, normal area imaging formats are not at all suitable. For example, standard NTSC image format (such as used in television) is less than 600 pixels wide, but this falls far short of the resolution required in many aerial reconnaissance applications. For example, reconnaissance cameras using roll film will often have resolutions of 20,000 equivalent pixels or more in width.
One potential advantage of electro-optic devices in reconnaissance systems is that data can be transmitted to remote locations, without awaiting physical transfer of film. Another potential advantage is that the delays and logistics requirements of emulsion processing can be avoided. Another potential advantage is that the output of an electro-optic imager is inherently better suited to interface to the image-recognition algorithms which may be developed in the future. Another potential advantage is that, as the capability to make images more understandable by preprocessing them advances, the output of electro-optic imagers will improve accordingly.
One well-known type of electro-optic imager is a charge-coupled device, or "CCD." A CCD is a semiconductor device wherein each imaging site is a potential well for minority carriers (normally electrons). Each potential well will collect electrons generated by photon absorption in its vicinity. The CCD output indicates the amount of charge collected in each well, and therefore the photon flux seen at each well.
Often a linear imager will be used, so that what is imaged at each instant is a strip on the ground. The motion of the platform sweeps this coverage along the ground, at the speed of the platform, to produce a large combined multi-strip image. In such systems, the use of an imager which has a large number of pixels will help to achieve high resolution in each strip (and therefore high cross-track resolution in the combined image), subject to the constraints of the optics. For example, it has been suggested that a linear CCD could be used as an electro-optic sensing element in an aerial reconnaissance system. See Rachel and Roberts, "Evaluation of the Electronic Wide Angle Camera System," at page 129 of the proceedings (designated volume 137) of the SPIE conference on Airborne Reconnaissance III (1978), which is hereby incorporated by reference. Note that this publication suggests that a linear CCD can be thought of as analogous to a scanning slit used to expose film.
Image Rotation
Published European Patent Application No. 0-127-194 (Application No. 84200649.6, filed May 8, 1984, claiming priority of French Application No. 8307911, filed May 11, 1983) shows an optical system mounted in a fixed nacelle on an aircraft. Rotatable elements permit pointing the field of view in any desired direction within a half sphere. This application recognizes that the rotation of the pointing elements will introduce a rotation into the image. This application further teaches that the apparent image rotation can be removed by use of a Pechan prism (shown as element P in the drawings).